1. Field of the Invention
The present invention relates to a method of fabricating a vertically tapered structure. More particularly, the present invention relates to a method of fabricating a vertically tapered structure using a shadow mask of a micro unit fabricated by a semiconductor process.
2. Description of the Related Art
Development of information society, which recognizes that organizations success depends on the ability to exploit information, has greatly changed human life. Optical communication technology has been recognized as one of the technologies that facilitates the development of the information society. If the amount of information increases in the future, optical communication is expected to occupy a dominant position in the communication of this information.
An optical device fabricated using SiO2 is mainly used in the current optical communication network. A refractive index of SiO2 is substantially 1.5 or so. Since SiO2 is used in an optical fiber that connects optical devices with each other, an area of an optical waveguide provided inside the optical device corresponds to an area of the optical fiber. Therefore, conventionally, optical loss at a junction area has not caused a big problem.
However, since the optical device fabricated using SiO2 has a relatively great volume, it has a drawback in that it is not suitable for the trend of ultra-small size and ultra-light weight. In this respect, efforts to use an optical device fabricated using silicon have been made. Silicon has a refractive index of 3.5 or so. The size of the optical waveguide can be reduced at several tens of microns(μm) unit if the optical device is fabricated using silicon. However, since an area of the optical waveguide fabricated using silicon is smaller than that of the optical fiber, a problem occurs in that an optical loss at the junction area increases.
In this case, the optical waveguide around the junction area can be fabricated in a vertically or laterally tapered structure to reduce the optical loss at the junction area. In other words, the optical waveguide around the junction area is tapered in a direction vertical or lateral to a waveguide junction direction, so that a shape of guided beam i.e., cross section can be increased. Thus, it is possible to improve the optical efficiency by controlling the optical waveguide so that it corresponds to the cross section of the optical fiber. Meanwhile, a laterally tapered structure can be easily fabricated using a pattern film. However, it is difficult to fabricate a vertically tapered structure.
FIG. 1 is a vertical sectional view illustrating a conventional method of fabricating an optical waveguide of a vertically tapered structure using a photoresist film. Referring to FIG. 1, a photoresist film 14 of a tapered structure is formed on a wafer of a silicon on insulator (SOI) structure in which Si 11, SiO2 12, and Si 13 are placed sequentially. The photoresist film 14 is then etched. Thus, a vertically tapered area can be fabricated as shown in FIG. 1. However, in case of the method depicted in FIG. 1, it is difficult to position the photoresist film 14 of the tapered structure on the wafer. Also, a problem occurs in that the size of the required tapered area and the angle of the tapered portion are not exactly aligned.
FIGS. 2A to 2C are vertical sectional views illustrating a conventional method of fabricating an optical waveguide of a vertically tapered structure using a shadow mask. Referring to FIG. 2A, a shadow mask 54 is separately fabricated and consists of a spacer layer 54a and a mask layer 54b. The fabricated shadow mask 54 is then joined on the wafer of a SOI structure in which Si 51, SiO2 52, and Si 53 layers are positioned sequentially. Next, as shown in FIG. 2B, ion beams are scanned into the SOI wafer while the SOI wafer is rotated in a horizontal direction. Thus, the Si layer 53 below the shadow mask 54 is partially etched to form a vertically tapered structure as shown in FIG. 2C.
Referring to FIGS. 2A to 2C, since the shadow mask 54 is fabricated using a mechanical etching process and a junction process, it is difficult to fabricate the shadow mask of a predetermined size or of a small size. Consequently, a problem occurs in that the tapered area depicted in FIG. 2B has a size (length) of several hundreds of microns. Accordingly, it is difficult to fabricate the shadow mask if the tapered area of several tens of microns is required.
Further, it is difficult to exactly align the laterally tapered structure and the vertically tapered structure in case of the method shown in FIGS. 2A to 2C. In other words, a junction error may occur when the shadow mask is fabricated in a vertically tapered structure. Such a junction error may serve as a significant defect in the optical waveguide of several tens of microns.